Why is Abscisic acid called a stress hormone?

Hint: Plants are subject to different forms of stress, such as drought, salinity, sun, cold, etc. The secreted compounds that control the various processes of physiology in plants are called the stress hormones. They are produced in minimal amounts. During stressful conditions, plant hormones play an essential role and enable plants to adapt to adverse environmental conditions.

Complete answer: Abscisic acid functions as an inhibitor of general plant growth and a plant metabolism inhibitor. ABA prevents germination of seeds. ABA promotes the closure of stomata in the epidermis and enhances plant resistance to different forms of stresses. It is also called the hormone of stress.
Abscisic acid is considered a stress hormone because environmental stresses such as drought, waterlogging, etc. enhance the hormone’s synthesis. In abiotic stress tolerance, it plays an important role. There are different developmental and biochemical mechanisms in which ABA plays a critical role: during high salinity, ABA promotes closure of stomata, water tension and decreases water loss through transpiration. To cause stomatal closure, ABA interacts with other phytohormones such as jasmonates, nitric oxide and signalling molecules.
ABA causes dormancy of the seeds and thus allows seeds to survive desiccation and other unfavourable growth conditions. For a longer time, seeds can be stored in a storage. During nitrogen deficiency and drought, ABA plays an essential role in root growth and alteration. It regulates the expression of genes necessary for the maintenance of root growth and water uptake.

Note:
Abscisic acid is involved in the synthesis of dehydrins, antioxidant proteins and osmoprotectants. ABA controls protein-encoding genes and lipid and storage protein biosynthesis. ABA plays a vital role in response to stress during the signal transduction pathway. By regulating stress-responsive genes, ABA stimulates long term development.